Computers, fax machines, printers and other electronic devices are routinely connected by communications cables to network equipment such as routers, switches, servers and the like. FIG. 1 illustrates the manner in which a computer 10 may be connected to a network device 30 (e.g., a network switch) using conventional communications plug/jack connections. As shown in FIG. 1, the computer 10 is connected by a patch cord 11 to a communications jack 20 that is mounted in a wall plate 18. The patch cord 11 comprises a communications cable 12 that contains a plurality of individual conductors (e.g., eight insulated copper wires) and first and second communications plugs 13, 14 that are attached to the respective ends of the cable 12. The first communications plug 13 is inserted into a plug aperture of a communications jack (not shown) that is provided in the computer 10, and the second communications plug 14 is inserted into a plug aperture 22 in the front side of the communications jack 20. The contacts or “blades” of the second communications plug 14 are exposed through the slots 15 on the top and front surfaces of the second communications plug 14 and mate with respective “jackwire” contacts of the communications jack 20. The blades of the first communications plug 13 similarly mate with respective jackwire contacts of the communications jack (not shown) that is provided in the computer 10.
The communications jack 20 includes a back-end wire connection assembly 24 that receives and holds insulated conductors from a cable 26. As shown in FIG. 1, each conductor of cable 26 is individually pressed into a respective one of a plurality of slots provided in the back-end wire connection assembly 24 to establish mechanical and electrical connection between each conductor of cable 26 and a respective one of a plurality of conductive paths (not shown in FIG. 1) through the communications jack 20. The other end of each conductor in cable 26 may be connected to, for example, the network device 30. The wall plate 18 is typically mounted on a wall (not shown) of a room of, for example, an office building, and the cable 26 typically runs through conduits in the walls and/or ceilings of the office building to a room in which the network device 30 is located. The patch cord 11, the communications jack 20 and the cable 26 provide a plurality of signal transmission paths over which information signals may be communicated between the computer 10 and the network device 30. It will be appreciated that typically one or more patch panels, along with additional communications cabling, would be included in the communications path between the cable 26 and the network device 30. However, for ease of description, in FIG. 1 the cable 26 is shown as being directly connected to the network device 30.
In the above-described communications system, the information signals that are transmitted between the computer 10 and the network device 30 are typically transmitted over a pair of conductors (hereinafter a “differential pair” or simply a “pair”) rather than over a single conductor. An information signal is transmitted over a differential pair by transmitting signals on each conductor of the pair that have equal magnitudes, but opposite phases, where the signals transmitted on the two conductors of the pair are selected such that the information signal is the voltage difference between the two transmitted signals. The use of differential signaling can greatly reduce the impact of noise on the information signal.
Various industry standards, such as the TIA/EIA-568-B.2-1 standard approved Jun. 20, 2002 by the Telecommunications Industry Association, have been promulgated that specify configurations, interfaces, performance levels and the like that help ensure that jacks, plugs and cables that are produced by different manufacturers will all work together. By way of example, the TIA/EIA-568-C.2 standard (August 2009) is designed to ensure that plugs, jacks and cable segments that comply with the standard will provide certain minimum levels of performance for signals transmitted at frequencies of up to 500 MHz. Most of these industry standards specify that each jack, plug and cable segment in a communications system must include eight conductors 1-8 that are arranged as four differential pairs of conductors. The industry standards specify that, in at least the connection region where the contacts (blades) of a plug mate with the jackwire contacts of the jack (referred to herein as the “plug-jack mating region”), the eight contacts in the plug are generally aligned in a row, as are the corresponding eight contacts in the jack. As shown in FIG. 2, which schematically illustrates the positions of the jackwire contacts of a jack in the plug-jack mating region, under the widely used TIA/EIA 568 type B configuration, in which conductors 4 and 5 comprise differential pair 1, conductors 1 and 2 comprise differential pair 2, conductors 3 and 6 comprise differential pair 3, and conductors 7 and 8 comprise differential pair 4. As known to those of skill in the art, conductors 1, 3, 5 and 7 comprise “tip” conductors, and conductors 2, 4, 6 and 8 comprise “ring” conductors.
Unfortunately, the industry-standardized configuration for the plug-jack mating region that is shown in FIG. 2, which was adopted many years ago, generates a type of noise known as “crosstalk.” As is known to those of skill in this art, “crosstalk” refers to unwanted signal energy that is induced onto the conductors of a first “victim” differential pair from a signal that is transmitted over a second “disturbing” differential pair. The induced crosstalk may include both near-end crosstalk (NEXT), which is the crosstalk measured at an input location corresponding to a source at the same location (i.e., crosstalk whose induced voltage signal travels in an opposite direction to that of an originating, disturbing signal in a different path), and far-end crosstalk (FEXT), which is the crosstalk measured at the output location corresponding to a source at the input location (i.e., crosstalk whose signal travels in the same direction as the disturbing signal in the different path). Both types of crosstalk comprise an undesirable noise signal that interferes with the information signal on the victim differential pair.
Various techniques have been developed for cancelling out the crosstalk that arises in industry standardized plugs and jacks. Many of these techniques involve providing crosstalk compensation circuits in each communications jack that introduce “compensating” crosstalk that cancels out much of the “offending” crosstalk that is introduced in the plug and the plug-jack mating region due to the industry-standardized plug-jack interface. In order to achieve high levels of crosstalk cancellation, the industry standards specify small, pre-defined ranges for the crosstalk that is injected between the four differential pairs in each communication plug, which allows each manufacturer to design the crosstalk compensation circuits in their communications jacks to cancel out these pre-defined amounts of crosstalk.
Unfortunately, due to the industry-standardized plug-jack interface, there generally is a spatial separation, and hence a corresponding time delay, between the region where the offending crosstalk is injected between conductive paths of the mated plug and jack and the region where the compensating crosstalk is injected. As hard-wired communications systems move to higher frequency signals (as is necessary to support higher data rate communications), this delay degrades the effectiveness of conventional crosstalk compensation schemes. In particular, conventional crosstalk compensation schemes couple signal energy from a second conductor of a disturbing differential pair onto a victim differential pair in order to cancel out the offending crosstalk that is generated when the first conductor of the disturbing differential pair couples energy onto the victim differential pair (e.g., in the plug-jack mating region). This compensation scheme works because the signals carried by the two conductors of the disturbing differential pair are phase-shifted by 180 degrees, and hence the signal energy coupled from the second conductor of the disturbing differential pair may be used to cancel out the signal energy coupled from the first conductor of the disturbing differential pair. However, because of the time delay between the points where the offending and compensating crosstalk signals are injected onto the victim differential pair, a phase shift will occur in the signal such that the offending and compensating crosstalk signals are not quite 180 degrees separated in phase. When higher frequency signals are used, the amount of this phase shift can become significant, which degrades the effectiveness of the crosstalk compensation.
In order to address the problem of phase shift at higher frequencies, so-called “multi-stage” crosstalk compensation schemes were developed, as disclosed, for example, in U.S. Pat. No. 5,997,358 to Adriaenssens et al. (hereinafter “the '358 patent”). Most high performance communications jacks that are in use today employ “multi-stage” crosstalk compensation circuits. With multi-stage crosstalk compensation, a first stage of “compensating” crosstalk may be provided (which has a polarity that is opposite the polarity of the offending crosstalk) that not only compensates for the offending crosstalk, but in fact over-compensates. Then, a second stage of compensating crosstalk is provided that has the same polarity as the offending crosstalk that cancels out the overcompensating portion of the first stage of compensating crosstalk. As explained in the '358 patent, the entire content of which is hereby incorporated herein by reference as if set forth fully herein, these multi-stage compensating schemes can theoretically completely cancel an offending crosstalk signal at a specific frequency and can provide significantly improved crosstalk cancellation over a range of frequencies.